Precipitation hardened tantalum base alloy



3,498,854 PRECIPITATION HARDENED TANTALUM BASE ALLOY Raymond W. Buckman, Jr., Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Continuation-impart of application Ser. No. 520,356, Jan. 13, 1966. This application Apr. 11, 1968, Ser. No. 720,453

Int. Cl. C22c 27/00 US. Cl. 148-325 Claims ABSTRACT OF THE DISCLOSURE A precipitation hardened tantalum base alloy having excellent fabricability and consisting of, by weight, at least one of the metals tungsten, molybdenum, and rhenium, the amount of tungsten being at least 5%, the molybdenum not exceeding 2%, and rhenium not exceeding 3 the total being at least 7% and not exceeding 10%, at least one of the metals zirconium and hafnium, the total zirconium equivalent being from 0.25 to about 0.75%,

hafnium being substitutable for zirconium up to a total of from 0.48 to 1.5% hafnium, from about 0.04 to 0.10% nitrogen, to provide an atom ratio of zirconium equivalent to nitrogen of 0.8 to 1.2, the balance being tantalum, and incidental impurities.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of my copending application Ser. No. 520,356 filed Jan. 13, 1966, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a precipitation hardenable tantalum base alloy which is characterized by high strength at elevated temperatures.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provision of section 305 of the National Aero nautics and Space Act of 1958, Public Law 85568 (72 Stat. 435; 42 U.S.C. 2457).

Description of the prior art Certain applications such as highly stressed rotating turbine components for space electrical power systems require a combination of enhanced strength and creep properties at elevated temperatures not available readily heretofore. The prior art discloses alloys, for example in Patent Nos. 3,075,840 and 3,183,085 relating to tantalum base alloys, comprising different alloying elements, having good strength and resistance to creep at elevated temperatures. However, these alloys are not precipitation hardenable, nor have they been susceptible to significant increases in strength and other improved properties by such heat treatment.

Moreover, most of the prior art alloys have had limited fabricability and in many cases have been incapable of extensive working and forging. Patent No. 3,243,290 discloses a tantalum base alloy including tungsten and zirconium and/or hafnium with at least 0.01% carbon and only residual amounts of nitrogen normally less than 0.005%, which accounts for tensile properties being inferior to the alloys of this invention.

1 United States Patent O "ice Likewise, a published report WANL-PR-M-004 of Westinghouse Astronuclear Laboratory by Ammon et al., entitled Pilot Production and Evaluation of Tantalum Alloy Sheet, dated June 15, 1963, discloses tantalum base alloys containing about 2 atom percent of hafnium for the amount of nitrogen present which adversely afiect the workability and also the strength properties in the presence of nitrogen.

It has been found that certain tantalum base alloys are precipitation hardenable by suitable heat treatment. In particular, this invention is directed to a tantalum base alloy having no appreciable amount of carbon above minimal residuals but having predetermined small amounts of nitrogen and zirconium or hafnium, or both of the latter, is responsive to precipitation hardening treatment, the alloy including selected amounts of certain solid solution hardening elements, and therefore results in tensile strength and elongation properties which are significantly superior to those of prior known tantalum base alloys.

Accordingly, it is an object of this invention to produce a precipitation hardened tantalum base alloy which has excellent strength at elevated temperatures and which is capable of excellent fabricability including forging applications.

Moreover, it is an object of this invention to provide a thermal treatment for a tantalum base alloy that accomplishes the foregoing objects and desiderata in a simple and effective manner.

SUMMARY OF THE INVENTION In accordance with the present invention wrought members have been produced from a cast member comprising a multicomponent tantalum base alloy comprising (a) certain proportions of tungsten, with molybdenum and rhenium being substitutable for a portion of the tungsten, and (b) a small amount of at least one metal selected from a group consisting of zirconium and hafnium, along with a small but critical amount of nitrogen, and the balance being tantalum except for incidental impurities and substantially no carbon.

DESCRIPTION OF THE PREFERRED EMBODIMENT After producing an ingot of the alloy, it is readily worked by hot working techniques such as forging, rolling and extrusion, followed by cold working if desired to selected shape including plates, rods, bars, or the like, and machined if desired. The fully shaped alloy member is solution annealed for example at 1900 C. to 2000 C. for an hour and finally precipitation hardened at temperatures of from 900 C. to 1325 C. for about an hour or more.

Tungsten is present in the alloy in an amount of from 7 to 10 weight percent. Molybdenum and rhenium may be substituted for part of the tungsten. Zirconium is present in an amount varying from 0.25 to 0.75 weight percent. Hafnium may be substituted on an atom-toatom basis for all or a part of the zirconium; namely, up to a total of from 0.48 to 1.50 weight percent of hafnium. The term zirconium equivalent is the weight of the zirconium plus half the weight of hafnium. Nitrogen is present in an amount of from about 0.04 to 0.08 weight percent to provide an atom ratio of zirconium equivalent to nitrogen of 0.8 to 1.2, and preferably about 1. The remainder of the alloy is entirely tantalum exprise a total of less than 0.1% of carbon, oxygen, silicon,

iron, sulfur, and the like. The impurities oxygen and carbon are preferably less than 100 parts per million.

As indicated a portion of the tungsten may be replaced, on an equal weight basis, by up to 2 weight percent of molybdenum and/ or up to 3 weight percent of rhenium to provide a modified alloy having at least about 5 weight percent tungsten. Molybdenum and rhenium in these proportionsare functionally equivalent to tungsten. The total tungsten equivalent (when molybdenum and/or rhenium are present) should not exceed weight percent in order to maintain satisfactory fabricability.

The optimum composition within the foregoing ranges comprises, in weight percent, about 8 percent tungsten, about 0.5 percent zirconium, about 0.08 percent nitrogen, (Zr/N atom ratio being about 0.96) and the balance is tantalum. This alloy composition may contain hafnium substituted for the zirconium up to a total of 0.95% hafnium.

The alloy may be melted by one of several procedures which ensure homogeneity and a minimum of contamination. For example, high purity tantalum together with the proper amounts of selected alloying elements may be fed into a conventional nonconsumable arc melting furnace containing an inert atmosphere, such as argon, or a vacuum. The desired nitrogen content is obtained by adding strips of tantalum nitride to the melt. The resulting ingot is then remelted, preferably by consumable arc melting, to achieve homogeneity, then it is hot worked to the desired shape. Material of similar quality can also be produced by consumable electrode arc melting or electron beam melting techniques. Moreover, the alloy may also be prepared by pressing powders, pellets or even shavings of tantalum and the selected alloy components together into a rod and consumably arc melting the rod. Nitrogen is added as tantalum nitride.

Several alloy compositions were prepared in accordance with the invention. Alloy A comprises in weight percent about 5.3 tungsten, about 1.56 percent rhenium, about 0.65 percent molybdenum, about 0.52 percent zirconium, about 0.08 percent nitrogen, and the balance essentially tantalum with small amounts of incidental impurities. Alloy B comprises in weight percent, about 8.1 percent tungsten, about 0.52 percent zirconium, about 0.08 percent nitrogen (the zirconium-nitrogen atom ratio being approximately 1), and the balance essentially tantalum with small amounts of incidental impurities. Hardness tests of samples of alloys A and B of the invention were made at room temperature and at various elevated temperatures listed in Table I.

TABLE I.-HARDNESS PROPERTIES Vickers Hardness After Aging 1 hr. at 0.)-

Alloy R.T. 700 800 900 1, 000 1, 100 l, 300 1, 400

1300 C. Above about 1325 C. the alloy forms a noncoherent optically observable precipitate and the hardness drops off rapidly.

j4.- i z 7 Samples of alloy A sheet material were tested at various heat treated conditions at several temperatures. The results are shown in Table II.

TABLE II.ELEVATED TEMPERATURE HARDNESS OF ALLOY A SHEET IN VARIOUS HEAT TREATED CONDITIONS Vickers Hardness at Temperature C.)

Heat

Treatment R.T. 816 982 1,093 1,204 383 194 171 155 126 411 217 184 161 149 365 203 156 141 112 From the above data it is evident that'alloy A of this invention has somewhat superiorhardness results when treated according to heat treatment 2 of TableII which involves heating the alloy for one hour at 2000 C. and then quenching it in helium gas after which it is heated for one hour at 1100 C. to cause precipitation hardening before cooling to room temperature. The hardness is determined at room temperature after heat treatment at the various temperatures listed in Table II. 4

The tensile properties for the alloy A are shown for tests conducted at various temperatures and are compared with corresponding properties for two commercially available high strength refractory metal alloys commonly known as T-222 (tantalum base) and TZC (molybdenum base) in Table III. The alloy T-222 has a composition of about 10 weight percent tungsten, about 2.5% hafnium, and about 0.01% carbon with the balance being tantalum and incidental impurities. The alloy TZC has a composition in weight percent of about 1.2% .titanium, about 0.25% zirconium, about 0.15% carbon, and the balance being molybdenum with incidental impurities.

TABLE III.TENSILE PROPERTIES Ultimate Tensile Elongation 0.2% Y S Strength 0 (13.5 1 (p.s.i.) (Percent) 1 Stress relieved one hour at 1,650 0. prior to test. d 2 grated in as-worked, prior 33% reduction, condition. Exhibited good no 11 y.

The tensile properties of alloy A of this invention are superior tothose of alloys T-222 and TZC when tested at room temperature, 2000,F., and 2400 F.

Alloys A and C of this invention were then tested to determine their creep properties at 1315 C.; for comparison with those of the alloys T-222 and TZC. The results are listed in Table IV below. The alloy C has a composition of 7.1% tungsten, 1.56% rhenium, 0.13% zirconium, 0.25% hafnium, about 0.04% nitrogen, and the balance being tantalum with incidental. impurities. The atom ratio of the total of hafnium and zirconium .to nitrogen is approximately 1. It was solution annealed at 2000 C. and not precipitation hardened before testing. Of course, precipitation hardening occurred during the test conducted at 1315 C.

TABLE IV.CREEP PROPERTIES OF ALLOY SHEET (TEST TEMPERATURE) 1,315 O.

Minimum 0. 2% Ultimate Test Total Creep Rate Yield Tensile Stress Duration Strain Percent Strength Strength Elongation (p.s.i.) (hrs) (percent) per hr.) (p.s.i (p.s.i.) (percent) 20,000 3 142 1. 25 0. 0094 20, 000 3 197 0. 21 0.0015 132, 700 140, 000 l6 14, 500 3 170 2. 11 0.008 Mo-TCZ 3 20, 000 Ruptured 1 Tested in overaged condition. 2 Tested in solution annealed condition. 3 Tests stopped before failure oceured.

Alloys A and C exhibit equal superior resistance to creep deformation as compared to the commercially available alloys T-222 and TZC under the conditions of test listed in Table IV.

The improved creep resistance exhibited by the alloys of this invention is attributable primarily to the dispersed secondary phase of the nitrides of hafnium and/ or zirconium.

The strengthening of the tantalum base alloys of this invention is achieved by the solid solution strengtheners tungsten, molybdenum, and rhenium. The precipitation hardening is due to the interaction of the reactive metal additions; that is, hafnium and. zirconium, with nitrogen, which forms a coherent precipitate within the tantalum alloy matrix. A coherent precipitate is a particle that has registry with the parent lattice and no discrete interface exists between the particle and the matrix, thereby preventing dislocation movement until very high stresses are applied. However, where carbon is present the benefits of optimum strength due to nitrogen derived by heat treatment are minimized because carbon forms a noncoherent precipitate located at a discrete interface between the particle and the matrix. A non-coherent precipitate does not strengthen to the same degree as a coherent precipitate. Thus, although the interaction of both carbon and nitrogen with zirconium and hafnium forms a dispersed secondary phase that can be termed precipitation strengthening, the precipitation kinetics for the carbides and the nitrides are different. Accordingly, it is critical to the alloys of this invention that they contain only up to 100 p.p.m. of oxygen and carbon as residuals. To be properly effective the amount of nitrogen should bev greater than 0.04 w/o.

The alloys are primarily suitable for forging applications, but they exhibit excellent fabricability as demonstrated by their being readily processed to sheet form. In view of the test results listed in the above tables the alloys of this invention are amenable to heat treatments to render the material in the overaged condition which permits fabrication using standard practices. Thereafter subsequent thermal treatment can be used to obtain a material having optimum strength properties.

The compositions of the alloys of this invention should be within the range listed above. The atomic ratio of the reactive metals, that is, hafnium and zirconium, to nitrogen should be maintained at substantially one for optimum resistance to creep deformation. Alloys of this invention having compositions in which the components are above the range listed are characterized by degradation of fabricability.

Inasmuch as oxygen removes hafnium and/or zirconium from solution, it is preferred that the amount of oxygen be restricted to an amount of 100 parts per million or less.

Accordingly, the tantalum base alloys of this invention provide for the attainment of superior tensile properties by precipitation hardening of a novel tantalum base alloy. The alloys have excellent hot fabricability and can be readily produced in sheet form. Subsequent precipitation hardening thermal treatment can be applied to the worked and shaped alloy to obtain optimum strength properties for such uses as turbine components and as high temperature testing machine parts. Moreover, the material is susceptible to cold rolling from room temperature to about 500 F. for finishing into plate, bar, rods, or wire or other forms even in solution annealed condition.

Various modifications may be made within the spirit of the invention.

What is claimed is: Y

1. A precipitation hardened tantalum base alloy consisting of, by weight, at least one metal selected from the group consisting of tungsten, molybdenum, rhenium and mixtures thereof, the amount of tungsten being at least 5%, the molybdenum, when present not exceeding 2%, the rhenium, when present not exceeding 3%, the sum of tungsten plus molybdenum plus rhenium being at least 7% and not exceeding 10%, at least one metal selected from the group consisting of zirconium, hafnium and mixtures thereof, the total zirconium equivalent being from 0.25% to 0.75%, from about 0.04% to 0.08% nitrogen, with the atom ratio of zirconium equivalent to nitrogen being within the range of from 0.8 to 1.2, and the balance tantalum with incidental impurities, the alloy being characterized by a microstructure consisting of coherent zirconium equivalent nitride precipitate particles having registry with the lattice matrix and being substantially free of discrete interfaces between the particles and the matrix.

2. The alloy of claim 1 in which there is from 7 to 10% tungsten, from 0.25 to 1.0% zirconium, from about 0.04% to 0.1% nitrogen, and the balance being tantalum with incidental impurities.

3. The alloy of claim 2 in which the atom ratio of zirconium to nitrogen is one.

4. The alloy of claim 1 in which there is about 5% tungsten, about 1.5% rhenium, about 0.6% molybdenum, about 0.5% zirconium, about 0.08% nitrogen, and the balance being tantalum.

5. The alloy of claim 1 in which there is about 7% tungsten, about 1.5% rhenium, about 0.1% zirconium, about 0.25% hafnium, about 0.04% nitrogen, and the balance being tantalum.

References Cited UNITED STATES PATENTS 3,194,697 7/1965 Chang 148133 3,243,290 3/1966 Clark et a1. 75174 3,379,520 4/1948 Chang et al 75l74 3,390,983 7/1968 Ammon et al. 75l74 OTHER REFERENCES Ammon et al., Pilot Production and Evaluation of Tantalum Alloy Sheet, WAN-LPR-M 004, June 15, 1963, relied on pp. 50, 51, 53, 55, 58, 62, 78, 79 and 80.

ASD-TDR62594, Investigation of Tantalum and Its Alloys, May 1963, relied on pp. 93, 95, 96, 101, 103,

104, 107 and 108.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE 7 CERTIFICATE OF CORRECTION Patent No. 3 1498 8514 Dated March 3 19-70 Inv hz) Raymond w, Buckman. Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, lines 68 and 69, change "2000F." to

" 2000C each instance SIG-NED AND F; 55H.

Tlilw Emu-E mm 1- mm, .m. I. om Manon of Patents 

